The present invention relates to a plasma processing method and a processing device suited to perform processing, such as etching, using plasma for metallic materials, such as aluminum, copper, platinum, and others, for insulating materials, such as silicone oxide, silicon nitride, and others, and for organic materials, such as a low dielectric constant film (low-k film), in the manufacturing process of semiconductor devices; and, more particularly, the invention relates to plasma processing equipment and a plasma processing method for processing especially fine patterns in a damage-free manner.
In the process of manufacturing semiconductor devices, such as a DRAM, microprocessor and ASIC, plasma processing using a weak ionizing plasma is widely used. In the plasma process, ions and radicals generated by plasma are irradiated on a wafer to be processed. As a refinement of semiconductor device advances, for plasma etching equipment used for processing wires, gate electrodes and contact holes, finer processing characteristics, high selectivity, high processing uniformity and low damage are required.
As a plasma source for etching equipment which has been used for many years, there is a parallel plate type plasma source. The parallel plate type plasma is called a CCP (capacitive coupled plasma) because the coupling of the plasma and power is capacitive. The parallel plate type plasma source has a comparatively simple device configuration and performs anisotropic etching using a comparatively high self bias generated when a blocking capacitor is inserted on the anode electrode side. However, as the pattern size of semiconductor devices becomes smaller, it is difficult to generate high-density plasma at a low pressure.
As a result, Japanese Patent Application Laid-Open 7-297175,IEM (ion energy modulation) indicates that, by applying high frequency waves of several tens MHz to the upper electrode of a narrow electrode parallel plate type reactor, a comparatively high-density plasma is generated, and by applying a bias of several hundreds of kHz to the lower electrode on which a wafer to be processed is disposed, the ion amount to be irradiated to the wafer to be processed is controlled. The pressure for stably generating plasma using IEM is high, such as several tens Pa to 5 Pa. However, to achieve further refinement of the pattern size, it is desirable to generate a plasma at a lower pressure.
In recent years, to produce inductively coupled plasma (ICP), a coil is wound around the side or top of a dielectric container and plasma is maintained by an induction field generated by supplying an alternating current to the coil. The ICP process can generate high-density plasma at a low pressure.
As a plasma source for generating high-density plasma at a low pressure, there is a magnetic field microwave type plasma source using electron cyclotron resonance (ECR). At a magnetic flux density of 875 G, the electron Larmor frequency becomes 2.45 GHz, and by resonance with the power frequency, plasma can be generated efficiently even at a low pressure.
Furthermore, a new plasma source for high-density plasma at a low electron temperature has been proposed. For example, Japanese Patent Application Laid-Open 9-321031 describes a UHF-ECR device using the UHF band of frequency of 300 MHz to 1 GHz, instead of a microwave for exciting the plasma. With respect to plasma excited by electromagnetic waves of the UHF band, the electron temperature is low, and the dissociation of the processing gas can be kept to an optimum condition, and, furthermore, the magnetic flux density for causing the ECR can be controlled to a low value, so that it is also advantageous in preventing damage.
Although the aforementioned ICP process can generate high-density plasma at a low pressure, the high density invites a disaster depending on the process, in that the dissociation of the processing gas advances extremely, and a problem arises in that a favorable selection ratio to the mask material or substrate material is not obtained.
In the aforementioned ECR, the high density and high electron temperature invite a disaster depending on the process, in that the dissociation of the processing gas advances extremely, and a problem arises in that a favorable selection ratio to the mask material or substrate material is not obtained. The problems of bias non-uniformity caused by the magnetic field and charging damage cannot be ignored.
When an etching process is to be performed using each of the aforementioned plasma sources, under the condition that the processing speed on the wafer surface is uniform, only the processing speed is uniform, and, although the plasma parameters, such as the plasma density, electron temperature, and plasma potential, are almost uniform, it cannot be said that they are strictly uniform.
The etching speed is determined by the balance between ions generated by the plasma, radicals, and reaction products generated by the etching. The density of the reaction products in the neighborhood of a wafer is always high at the center of the wafer, if the etching speed on the wafer is uniform, so that, to cancel it, the ion distribution, under the condition that the processing speed is uniform, is always crowning.
As mentioned above, to make the processing speed uniform, the plasma distribution is adjusted to be slightly non-uniform. Even if the plasma density is uniform, the electron temperature and plasma potential may become naturally non-uniform.
Meanwhile, in order to achieve refinement of semiconductor devices, the minimum processing size at the time of manufacture of the devices is becoming smaller year by year, and in correspondence to this, the thickness of the oxide film of the gate becomes thin, for example, to about several nm.
When the oxide film of the gate becomes extremely thin, the withstand voltage of the oxide film becomes low, so that the semiconductor device becomes very sensitive to damage when the oxide film is exposed to plasma. One example of charging damage caused by plasma is macro damage. This phenomenon will be explained with reference to FIGS. 14A to 14C.
At the time of etching, after the plasma is ignited, a high frequency bias is generally applied to a wafer. Ions are pulled in the wafer by the high frequency bias, and, hence, etching of high anisotropy is realized. When the high frequency bias is applied, depending on the difference in the displacement between the electrons and the ions, as shown in FIG. 14A, a self bias Vdc is generated on the wafer.
As described previously, the etching process is performed under the condition that the processing speed on the wafer surface is uniform. However, only the processing speed is uniform, and it cannot be said always that the plasma parameters are uniform. Therefore, the aforementioned self bias Vdc, as shown in FIG. 14B, may be different depending on the position on the wafer.
When the self bias voltage Vdc on the wafer surface is more than the withstand voltage of the oxide film of the gate, as shown in FIG. 14C, the current flows through the plasma as a part of the circuit and the device breaks. Namely, macro damage is generated.
FIG. 15 shows an example of the current-voltage characteristic of an oxide film of the gate. Although the characteristic is different depending on the conditions, such as the thickness of the oxide film of the gate, when a voltage between 7 V and 8 V or so is applied to the oxide film of the gate, the oxide film of the gate is subjected to a dielectric breakdown. When the thickness of the oxide film is made thinner, the withstand voltage will be naturally lowered. When the difference in the self bias potential on the wafer surface is more than 7 V, macro damage is generated.
The non-uniformity of the self bias as shown in FIG. 14B can be seen often when a magnetic field is applied to a plasma. The reason for this is that the motion of electrons is restricted by the magnetic field, and the impedance of the plasma is different between the crossing direction of the magnetic field and the parallel direction with the magnetic field. This will be explained by referring to a schematic view.
FIG. 16 shows an equivalent circuit of plasma viewed from a high frequency bias. The magnetic field, as shown in FIG. 16, enters almost perpendicularly to the wafer. The current flowing through the plasma is mostly comprised of electrons. However, when there is a magnetic field, electrons make the Larmor rotation around the lines of magnetic force, so that the motion across the lines of magnetic force is difficult. Therefore, the impedance of plasma in the crossing direction of the lines of magnetic force is larger than that in the parallel direction with the magnetic field. Therefore, a potential difference is apt to be generated in the crossing direction of FIG. 16, and it appears as a non-uniformity of the bias and causes macro damage. A mechanism for causing this non-uniformity of the bias will be explained hereunder.
FIG. 13A is a diagram showing the concept of a conventional processing method, for example, in which the current path of a high frequency bias applied to a wafer is not limited to the opposing electrode. Generally, when a high frequency bias is applied to a lower electrode, a negative self bias is generated. The reason for this is that the side wall of the processing chamber operates as a ground (electrode) for the lower electrode, and the area of the ground (electrode) becomes larger than the area of the lower electrode. Since the current path of the high frequency bias is not limited to the opposing electrode and the outer periphery of the wafer is adversely affected by the ground of the side wall of the processing chamber, the effective ground area as viewed from the high frequency bias is larger at the outer periphery of the wafer than in the center part of the wafer. Namely, the impedance up to the ground as viewed from the high frequency bias is non-nuiform in the wafer surface, and the impedance, in other words, the negative self bias, is larger on the outer part of the wafer surface than on the inner part.
When the magnetic field is applied perpendicularly to the processing chamber, the plasma impedance in the crossing direction of the magnetic field is large. When a high frequency bias voltage is applied to such a magnetized plasma, unless the current path of the high frequency bias is limited to the opposing electrode, at the center part of the wafer, the high frequency bias current flows only through the opposing electrode, and in the outer periphery of the wafer, the high frequency bias current also flows through the opposing electrode and side wall of the processing chamber. When the high frequency bias current flows through the side wall of the processing chamber, as mentioned above, the plasma impedance in the crossing direction of the magnetic field is large, so that a larger voltage drop is generated. Namely, as shown in FIG. 13B, the self bias generated at the center of the wafer is different from the self bias generated in the outer periphery of the wafer, and, as shown in FIG. 14B, the absolute value of the self bias potential in the outer periphery of the wafer is larger. As a result, the self bias potential on the wafer becomes non-uniform, and, as shown in FIG. 14C, a current flows through the device via the plasma. As a result, the processing method shown in FIG. 13A may cause macro damage easily.
Although non-uniformity of the plasma impedance due to the magnetic field is assumed for purposes of explanation, even if no magnetic field is applied, the same phenomenon may be generated due to non-uniformity of the electron temperature and other factor. As described previously, under the condition that the processing speed is uniform, the plasma is not always uniform, and when a radial distribution of the plasma impedance is generated due to a spatial non-uniformity, such as the plasma density and electron temperature, a non-uniformity of the bias and macro damage results.
An object of the present invention is to eliminate the aforementioned bias non-uniformity and to provide a plasma processing method which is free of macro damage and plasma processing equipment which is free of macro damage.
Another object of the present invention is to provide a plasma processing method and processing device for processing even fine patterns having a processing size of 0.2 mm or less without damage.
A characteristic of the present invention is that, in plasma processing equipment having a vacuum processing chamber, a plasma generation means, a stage for loading a wafer to be processed in the vacuum processing chamber, an opposing electrode having an area almost equal to or wider than the aforementioned wafer, which is installed opposite to the stage, and a bias power source for applying a high frequency bias to the wafer, a current path correction means is provided for correcting the part of the current path in the neighborhood of the outer periphery of the wafer among the high frequency current path by the high frequency bias so as to look toward the wafer opposing surface of the opposing electrode.
Another characteristic of the present invention is that, in plasma processing equipment having a vacuum processing chamber, a plasma generation means, a stage for loading a wafer to be processed in the vacuum processing chamber, an opposing electrode having an area almost equal to or wider than the aforementioned wafer, which is installed opposite to the stage, a ground, and a bias power source for applying a high frequency bias to the wafer, an impedance adjustment means is provided for making the impedance up to the ground as viewed from the high frequency bias almost uniform on the wafer surface.
Still another characteristic of the present invention is in the provision a plasma processing method performed by plasma processing equipment having a vacuum processing chamber, a plasma generation means, a stage for loading a wafer to be processed in the vacuum processing chamber, an opposing electrode having an area almost equal to or wider than the aforementioned wafer, which is installed opposite to the stage, and a bias power source for applying a high frequency bias to the wafer and including a current path correction means for correcting the current path part in the neighborhood of the outer periphery of the wafer among the high frequency current path by the high frequency bias so as to look toward the wafer opposing surface of the opposing electrode, which method comprises a step of introducing processing gas into the vacuum processing chamber, a step of turning power on, igniting a plasma, applying a high frequency bias to the wafer, and generating a uniform self bias potential on the wafer when the high frequency bias is applied by the current path correction means, and a step of processing the wafer using the plasma.
Macro damage is caused by non-uniformity of a self bias in the wafer surface which is generated when a high frequency bias is applied so as to attractions to the wafer after the plasma is ignited. Therefore, even if the plasma has a slight non-uniformity, when the self bias is made uniform according to the present invention, macro damage can be prevented.
Namely, according to the present invention, since no current flows through the device via the plasma, macro damage can be prevented. Furthermore, according to another characteristic of the present invention, the self bias potential on the bias becomes uniform and macro damage can be prevented.